The invention relates to a spectrometer wherein a spectrum is scanned by the rotation of a dispersive element. The wavelength or the wavenumber may be selected as the input variable. Depending on the selected input variable, the dispersive element will be rotated by such an angle that the radiation corresponding to the preselected wavelength or wavenumber, respectively, will be incident on the exit slit of the spectrometer. Usually, the recording strip of a recorder is moved in the abscissa direction in linear dependence on the input variable, i.e. either on wavelength or the wavenumber, and the associated absorption or transmission is recorded in ordinate direction.
Generally, the relationship of the input variable to the respectively associated rotational angle of the dispersive element will be non-linear. When a grating is employed as the dispersive element, said non-linear relationship will be represented by an angular function. If the wavelength serves as the input variable, the sine of the rotational angle will be proportional to the input variable. If the wavenumber serves as the input variable, the proportionality will exist between the input variable and the reciprocal of the sine of the rotational angle.
In known apparatus the non-linear relation was realized by lever gears, the input adjustment of which was linearly dependent on the input variable and the output adjustment of which produced a rotational angle depending on the input variable, in accordance with the respectively desired function. An example of such mechanical equipment is shown in German Auslegeschrift No. 1,159,663.
When a prism is employed as the dispersive element, an empirically determined function, which cannot be represented analytically, will result as the relationship between the input variable and the rotational angle. As a result the function has to be stored on a control cam plate, which is scanned by a sensing lever.
All such mechanical gear means require high precision and accurate adjustment and thus are expensive to manufacture. Furthermore, there are difficulties in designing a spectrometer in such a way as to enable the spectrum to be scanned, selectively, in dependence on either a linearly variable wavelength or a linearly variable wavenumber.
It is known to use a stepping motor as the adjusting motor for the dispersive element. However, when the stepping motor was controlled, selectively, in accordance with either the wavelength or the wavenumber, mechanically operated gear means had to be provided intermediate the stepping motor and the dispersive element (German Offenlegungsschrift No. 2,513,225).
U.S. Pat. No. 3,976,378 shows a spectrometer in which the recorder and the mechanism for rotating the prism are each driven by a separate stepping motor. The pulses for advancing both of the stepping motors are generated by a common timing pulse generator with a frequency divider, providing for the two different rotational speeds. However, these two speeds are in a well-defined ratio with respect to each other. The mechanism for rotating the prism comprised a cam plate, which transformed the rotational angle of the stepping motor, varying linearly with the desired wavelength, to the prism, the rotation of which varied non-linearly with the wavelength.
Furthermore, a computer controlled spectrometer is known, which included a rotatable dispersive element, a stepping motor for varying the angular position of said dispersive element, a program transmitter for controlling said stepping motor, a timing pulse generator, and a recording instrument (Applied Spectroscopy, Vol. 28, No. 1 (1974), Pages 45 to 51).
In said known spectrometer the spectrum was scanned step-wise in equal steps of 1 cm.sup.-1 with each step being within a preselected wavenumber range. This operation was controlled by the program transmitter by adjusting the monochromator to the initial wavelength, determining the base line and, in each step, adjusting the multiplier voltage and taking the measurement. After each measurement a test was performed to determine whether or not the terminal wavelength of the wavelength range to be scanned had been reached. If not, the monochromator was advanced by one step in wavenumbers and the subroutines "Adjusting multiplier voltage" and "Measuring" were repeated.
Advancement of the monochromator by equal steps in wavenumber was achieved by means of a stepping motor. Each step thereof had to correspond to a constant step of 1 cm.sup.-1 in wavenumbers. Since, as stated in the publication, the relationship between the wavenumbers and the number of steps of the stepping motor was non-linear, it was necessary to provide non-linear gear means intermediate the stepping motor and the dispersive element in order to realize continuously equal steps in wavenumber.